It isn't because their bodies are not touching other than the man holding one hand and the other on the woman's waist not touching hip, buttock or too close to the bosom.
I don't think it is, but some people do. I saw one website that claimed that any dancing between a man and woman was sinful on the grounds that dancing between men and women was never mentioned in the Bible. Under that logic, anything that's not mentioned in the Bible should be destroyed or eliminated.
If that’s the logic that’s being followed, then almost everything is sinful. Eg, watching movies, buying stock, flying in an airplane, driving a car, etc.